1. Field of the Invention
The present invention relates to the field of laser techniques, particularly to methods and apparatus for transforming and steering laser beams, especially in the field of medical applications, such as opthalmic, surgical, and therapeutic treatments.
2. Description of Prior Art
At the present time, laser beam techniques find wide applications in surgical operations, such as incisions, tissue attachments, coagulating, etc. Typically, a laser apparatus consists of two laser beam systems, which operate on different wavelengths. One laser beam system is used to generate a power beam which performs the operation itself and operates on an invisible wavelength. The second laser beam system, which is aligned with the first one, is used for indicating the power beam and therefore operates on a visible wavelength. In other words, the surgeon using the apparatus guides the power beam in the operation area by observing the position of the visible guide beam in such area. The visible beam has lower energy than the level required for treating an object, while the power beam has a level of energy capable of performing the operation. Both beams are guided through an adjustable articulating arm or an optical fiber link, which delivers the beams to a focusing unit. One typical system of this type is decribed in U.S. Pat. No. 4,917,083 to J. Harrington and M. Clancy, 1990.
At the present time laser beam focusing systems use optical lenses as their main focusing elements. Such a focusing system, which is known as a telescope, usually consists of a tubular housing, which contains a number of lenses arranged on the general optical path and intended for focusing both laser beams on the operation area, see Micromanipulator Model 5000, produced by Coherent, Inc., Palo Alto, Calif. In operation, the user, depending on the type of the procedure, must select a proper distance to insure either focusing or defocusing conditions.
It is known, however, that all lens type systems have refraction indexes dependent upon the wavelength of the laser beam used in the system. This causes problems, which practically cannot be solved to full extent. More specifically, the power, or invisible beam is in a far infrared wavelength range, i.e., 10.6 microns, and the guide, or visible beam has a wavelength of 0.632 micron. This dependency is known as a chromatic aberration.
Aberration is a failure of a lense system to produce point-to-point correspondence between an object and its image. Chromatic aberration occurs in optical systems with several wavelengths. In other words, two coaxial laser beams of different wavelengths incident on the same point are refracted by the system to different degrees and thus cannot be focused upon the same target point.
The same phenomenon causes different absorptions of laser beams, which also leads to substantial energy losses and inaccuracy. For example, the latest laser beam steering apparatus of the Model 5000 micromanipulator produced by Coherent, Inc., which is one of the most accurate devices known in the art, specifies a minimum focusing spot diameter of 0.4 mm and maximum defocusing diameter of 8.5 mm. This means that the existing apparatus is not sufficiently accurate and therefore, unreliable and unsuitable for critical surgical procedures for which a surgeon has to be absolutely confident in that both beams are coincident in the same point and have the same spot size.
Moreover, any laser beam delivery system based on the use of optical lenses is unequivocally dedicated only to one predetermined laser source wavelength. This means that each time the user wants to change the laser wavelength (for example, for changing type of a surgical procedure) such user has to replace the laser beam steering apparatus. All of the above will not allow the surgeon to switch from one type of laser source to another without changing the original setup (for which the surgeon must purchase an additional optical delivery system for a specific wavelength). Also the surgeon cannot upgrade the procedure because of the spot size, even though the surgeon uses the same laser source.
Another disadvantage of the conventional laser beam delivery system is complicated construction, as it consists of a plurality of lenses and lens adjustment mechanisms. The complicated construction is difficult to manufacture and has a high cost because of the use of highly expensive optical substrates and coatings. The high price also results from the necessity of the purchasing and storing a set of laser sources and steering apparatuses in conjunction with various surgical procedures which may be required.
Therefore, there is a demand in the field of medical laser beam technique for a less expensive, versatile, and adjustable laser steering system.